Lifting of heavy objects, such as fuel assemblies, is a necessary activity performed for safe and continuous operation of nuclear power plants. Lifting of such nuclear fuel assemblies is often strictly regulated with varying precautions being undertaken prior to actual load lift. Typical precautions include, for example, ascertaining the weight of the assembly, determination of a path that the assembly will take during the lift, identification of critical and/or safety sensitive nuclear related components which may be jeopardized during the lift, and evaluation of potential damage in the event of a load drop.
Fuel assemblies provided for boiling water nuclear reactors are composed of a plurality of fuel rods which are supported at a lower end by a lower nozzle or end fitting. A top nozzle or end fitting is located above the plurality of rods. Water rods are interspersed in the plurality of fuel rods to help channel coolant flow and moderate the nuclear reaction. Between the upper and lower nozzles or end fittings, a plurality of spacer grids are positioned at intervals to provide lateral support and prevent potentially destructive side to side movement of individual fuel rods. The fuel assembly top nozzle and bottom nozzle may be configured to aid in channeling coolant flow through the assembly during operation, the bottom nozzle accepting coolant flow and the top nozzle discharging coolant from the assembly in the case of a conventional boiling water reactor fuel assembly. In this typical configuration, the weight of the fuel rods is supported by structures such as an internal water channel, the water rods, inert rods or tie rods during fuel assembly load lift.
Certain areas in a nuclear power plant are more safety sensitive and require extremely thorough procedures to ensure continued health and safety of the public at large. Such safety sensitive areas include, among other areas, the nuclear reactor itself and the fuel pool. The presence of potentially large amounts of radioactive materials in these areas, as well as vital cooling systems, requires the utmost care when lifting is performed. The resulting required safety in these areas necessitates additional checks before load lift. A typical check usually involves, for example, inspecting the structural components of the polar crane or a fuel handling crane. In addition to inspecting the crane, the actual load lifted (i.e. the fuel assembly itself) is inspected and evaluated.
Nuclear plant operating experience indicates that when typical materials used in nuclear reactors are exposed to radiation over 62 Mwd/kgU, these materials may start to degrade. Specific experience shows a potential for stress corrosion cracking, hydrogen embrittlement and irradiation hardening which may challenge the structural integrity of both tie rods and water rods as well as other components used in lifting a fuel assembly with current designs. Currently, structural members used for a load lift of boiling water reactor fuel assemblies may be composed of a zirconium alloy with a thickness of less than 0.03 inches. Although weight reduction is achieved, the minimal corrosion allowance provided for these members from corrosion is a serious potential problem during load lift. As a typical potential problem, water rods, which may also be used to carry the fuel assembly load, are normally exposed to coolant on both inner and outer surfaces, which in turn increases the possibility of corrosion on these surfaces. Additionally, as a result of this exposure, hydrogen embrittlement of the water rod may occur. The water rod hydrogen concentration may approach 500 to 700 ppm, thereby requiring a reduction in load carrying capability for the assembly components. If these problems occur, current practices in the nuclear industry require costly and time consuming alterations to fuel assemblies which are exposed to radiation at or near this level of radiation, which are mechanically damaged, or for fuel assemblies which may have hydrogen embrittlement. To correct these problems, complete disassembly of the fuel assembly and reconstitution of the fuel assembly may be needed with new parts installed in the fuel assembly.
New (unirradiated) fuel assemblies also present other load lift difficulties. New fuel assemblies must be lifted by a special lifting device to allow for sufficient structural lift capacity over critical plant/safety sensitive areas. These special lift devices often provide complicated structures which may include connecting bolts, bayonet mounts, compression springs and washers to transfer the weight of the assembly from the fuel assembly to the crane.
There is a need to provide a lifting support for a boiling water nuclear fuel assembly to eliminate complicated structural support mechanisms currently used in existing nuclear fuel assembly configurations.
There is a further need to provide a lifting support for a fuel assembly for new (unirradiated) fuel handling operations.
There is a still further need to provide a configuration that will allow repair of fuel assemblies which exhibit stress corrosion cracking problems, hydrogen embrittlement, irradiation hardening or other similar load path defects without incurring costly and time consuming alterations to the fuel assembly.